1. Field of the Invention
The present invention relates to a measurement apparatus, exposure apparatus, and device manufacturing method.
2. Description of the Related Art
In the lithography process to manufacture a device such as a semiconductor device, an exposure apparatus is used to transfer the pattern on a reticle (original) onto a wafer (substrate) coated with a resist. Along with the trend toward integrated circuits with a higher density and higher degree of integration, the exposure apparatus must have higher resolution, and accordingly the wavelength of the exposure light is becoming shorter. More specifically, as the light source of the exposure apparatus, use of a KrF excimer laser (wavelength: 248 nm) and an ArF excimer laser (wavelength: 193 nm) has become popular to replace a mercury lamp.
An increase in numerical aperture (NA) of the projection optical system of the exposure apparatus can also increase the resolution. Hence, an immersion exposure apparatus has been proposed in which a liquid with a refractive index larger than 1 fills the space between the projection optical system and a wafer to increase the numerical aperture (NA).
As the resolution of the exposure apparatus increases, strict demands have arisen for measurement and correction of the aberration of the projection optical system of the exposure apparatus. For example, regarding the influences of a slight change in lens position occurring during conveyance of the exposure apparatus and of a change in optical characteristics caused by heat during exposure, they must be grasped accurately and adjusted. For this purpose, after the projection optical system is mounted in the exposure apparatus main body, it is indispensable to measure the optical characteristics of the projection optical system in the exposure apparatus and to adjust them to an optimum state. The optical characteristics do not depend on polarization to be described later but represent the difference in phase from an ideal state, in the same manner as conventional wavefront aberration, and will be described as “unpolarization aberration” hereinafter.
As the numerical aperture (NA) increases, the influence of polarization becomes critical. This leads to the study of a scheme of controlling the polarization of the exposure light. Even when an illumination system controls the exposure light to a desired polarization state, however, distortion of a lens caused by holding the lens or the heat in the lens, use of a birefringent material such as fluorite, reflection of a mirror, and the like may change the polarization state within the projection optical system. Therefore, the necessity to accurately grasp a change in polarization in the projection optical system also increases. As the optical characteristics due to the birefringence of the projection optical system depend on the polarization of an incident beam, they will be described as “polarization aberration” hereinafter. Polarization aberration includes retardation caused by birefringence, and its azimuth. Polarization aberration is expressed in various manners, for example, by the Stokes parameter, Jones matrix, Jones vector, and Mueller matrix, in addition to retardation and azimuth.
In this manner, as the performance of the exposure apparatus improves, demands have arisen to measure unpolarization aberration not dependent on polarization and polarization aberration dependent on polarization quickly and accurately as the optical characteristics of the projection optical system.
As the scheme of measuring the unpolarization aberration and polarization aberration of an optical system, Japanese Patent Laid-Open Nos. 2-116732, 2002-71515, and 2004-257854 propose an interferometer for measurements using interference of a reference wavefront and detection target wavefront. PCT (WO) 2003/028073 proposes use of an aberration measurement unit and a measurement unit which roughly estimates wavefront aberration accompanying birefringence in aberration measurement of a projection optical system. Japanese Patent Laid-Open No. 2005-116732 proposes a method of calculating the Mueller matrix as the polarization information on a projection optical system. Japanese Patent Laid-Open Nos. 2004-61515 and 2006-237617 propose a scheme of measuring the polarization aberration of a projection optical system using an interferometer.
The conventional schemes described above, however, have various problems. The interferometer for measurements disclosed in Japanese Patent Laid-Open Nos. 2-116732, 2002-71515, and 2004-257854 is not appropriate for measurement in the exposure apparatus because its exposure light source has a very short coherent length, in addition to its long, complicated optical path as a non-common path, so that the influence by vibration is non-negligible.
The method disclosed in PCT (WO) 2003/028073 proposes measurement by means of an aberration measurement unit and polarization conversion measurement unit using only two orthogonal linearly polarized beams. With only two orthogonal linearly polarized beams, the retardation and azimuth of birefringence as the polarization aberration of the projection optical system cannot be calculated accurately. According to the method disclosed in PCT (WO) 2003/028073, both the aberration measurement unit and polarization conversion measurement unit respectively have CCDs serving as image sensors. In other words, this method requires two CCDs, which is costly. As the CCDs are inserted in and removed from the exposure apparatus, their positions with respect to the optical axis of the exposure light change. This may hinder accurate measurement.
In addition, the two measurement units must be aligned before measurement. Depending on the alignment accuracy, the correspondence between unpolarization aberration and polarization aberration can change, disabling accurate phase measurement.
Further, lens groups are arranged in the two measurement units, each also requiring optical axis alignment with the optical axis of the measurement apparatus including the projection optical system. Since the effect of an alignment error in the arrangement of the lens group can be extremely big, high accuracy in alignment is required. Therefore, it is necessary to carry out calibration for correcting the alignment error whenever the unit is exchanged, and measurement time is doubled. In addition, this alignment error can leads to deterioration in accuracy, and become a big error-coming factor when highly accurate measurements are asked for.
The method disclosed in Japanese Patent Laid-Open No. 2005-116732 employs a general method of detecting the polarization state of a detection target. According to this method, after being transmitted through a detection target, light passes through a waveplate, polarizer, and image sensor arranged in this order. The waveplate is rotated to detect a change in intensity, thereby detecting the polarization state of the detection target. Although the Mueller matrix is calculated accurately as polarization aberration, this reference discloses nothing concerning measurement of unpolarization aberration.
Japanese Patent Laid-Open Nos. 2004-61515 and 2006-237617 propose a scheme of measuring the polarization aberration of a projection optical system by using an interferometer. These references also disclose a method that employs shearing interference. However, the scheme disclosed in Japanese Patent Laid-Open Nos. 2004-61515 and 2006-237617 does not provide a system that can measure reference polarization aberration in the apparatus. Hence, the system cannot cope with a real-time change of information on the reference polarization aberration, and thus cannot detect polarization aberration information quickly and accurately. Even if the apparatus uses reference polarization aberration information which is measured separately, the information is different from the reference polarization aberration information of the apparatus due to influences such as how the projection optical system incorporated in the exposure apparatus is held, deformation of the projection optical system by its weight, thermal change by exposure, and the like. Therefore, polarization aberration cannot be calculated accurately.
More specifically, with the conventional scheme, unpolarization aberration and polarization aberration (retardation and azimuth of birefringence) which are the optical characteristics of the projection optical system cannot be measured in the exposure apparatus simultaneously, in real time, quickly, and accurately.
In recent years, aberration (exposure aberration), or unpolarization aberration and polarization aberration that change (change over time) in a real time as the optical system generates heat upon exposure, has become a major issue. As micropatterning progresses, the tolerances of unpolarization aberration and polarization aberration decrease, and the required measurement accuracy is becoming stricter and stricter. Therefore, it is very important to measure the unpolarization aberration and polarization aberration of the optical system, with a measurement apparatus being incorporated in the exposure apparatus, quickly (in a real time) and accurately, and to perform real time correction in accordance with the measurement result.